The present invention relates to a method of control of an air-fuel intake system for an internal combustion engine, and more specifically relates to a method of control of an air-fuel intake system for an internal combustion engine which includes an intake port construction including both direct and helical intake passages, wherein the relative amounts of intake flow which pass through said direct intake passage and said helical intake passage can be varied, and which further includes a fuel injection system in which liquid fuel (e.g. gasoline) is directly squirted into the inlet ports of the engine by a fuel injection nozzle.
There are some types of variable swirl intake port construction for an internal combustion engine, which have been developed by the present inventors in the works of the assignee of the present application previously to the development of the present invention, for the inventive concepts of which Japanese Patent Application Serial Nos. 56-51149 (published as Japanese Patent Laying-Open Publication No. 57-165629) and 56-120634 (published as Japanese Patent Laying-Open Publication No. 58-23224) were filed previously to the filing of the Japanese Patent Application relating to the present invention of which priority is being claimed in the present application, and for said inventive concepts of which it is known to the present inventors that U.S. patent application Ser. Nos. 341,911 and 404,145 have been filed claiming the priority of the above identified Japanese patent applications, which incorporate two intake passages formed as leading to the opening past the intake poppet valve into the combustion chamber of the engine, one of said passages leading substantially straight to said opening, and the other leading in a curved or helical path to said opening. The first or the straight one of said intake passages is controlled by an intake control valve mounted at an intermediate position therealong, so that its effective flow resistance is variable; and thus the relative amounts of intake flow which pass through said direct intake passage and said helical intake passage can be varied. In other words, when said intake control valve is closed to the maximum extent (i.e., in general, is fully closed), then a maximum proportion of the intake flow sucked in by the combustion chamber through the intake port construction is sucked in through the helical intake passage and a minimum proportion of said intake flow is sucked in through the straight intake passage, so that as a whole a maximum amount of swirling is imparted to the intake gas sucked into the combustion chamber. On the other hand, when said intake control valve is closed to the minimum extent (i.e., in general, is fully opened), then a minimum proportion of the intake flow sucked in by the combustion chamber through the intake port construction is sucked in through the helical intake passage and a maximum proportion of said intake flow is sucked in through the straight intake passage, so that as a whole a minimum amount of swirling is imparted to the intake gas sucked into the combustion chamber. Such a type of variable swirl intake port construction for an internal combustion engine is schematically shown in FIGS. 1 and 2 of the accompanying drawings, and will be more fully explained in the portion of this specification entitled "DESCRIPTION OF THE PREFERRED EMBODIMENT".
When the intake flow of an internal combustion engine is imparted with a strong swirling, as in the above described case when the aforesaid intake control valve in the straight intake passage is closed as far as possible so that most or all of the intake flow of the engine passes through the helical intake passage, then the apparent flame propagation speed is increased, and it is possible to operate the engine with a very lean mixture, i.e. with a high air/fuel ratio. Further, strong intake swirling helps with stable idling of the engine, so that, other things being equal, the idling speed can be set very low, even when the air/fuel ratio of the mixture being supplied to the engine fluctuates somewhat. On the other hand, the intake volumetric efficiency is reduced, especially during high load engine operation. But in the case when no or very little intake swirling is provided, as in the above described case when the aforesaid intake control valve in the straight intake passage is opened as far as possible so as to combine the flow through said straight intake passage with the flow through the helical intake passage, then the apparent flame propagation speed is lower and the engine cannot be operated on mixture of such a low air/fuel ratio, and the idling speed cannot be set so low and the idling is not so stable, but on the other hand the intake volumetric efficiency is much higher. Thus, in the above-identified prior applications, a general form of control method of the intake control valve has been to close it in the engine operational region from low to medium load, so as to provide high swirling for the gases entering the combustion chamber, and so as to eliminate flow through said straight intake passage and to concentrate flow in said helical intake passage, while on the other hand in the engine operational region from medium to high load it has been practiced to provide low swirling for the gases entering the combustion chamber by opening said intake control valve, so as to promote flow through said straight intake passage while reducing the concentration of flow in said helical intake passage, and so as to increase volumetric efficiency.
Now, this basic form of control method for the intake control valve as described above has the advantages as outlined, but, in the case that such a construction is applied to an internal combustion engine with a fuel injection system in which liquid fuel is directly squirted into the inlet ports of the engine by fuel injection nozzles, the following shortcoming occurs. As the engine is being started up from the cold condition, the intake control valve is opened up fully; and while the engine is operating in the cold condition, i.e. just after the starting up of the engine and while it is warming up, then as usual the intake control valve is closed if the engine load is low and is opened if the engine load is medium to high. This presents no problem with an engine utilizing a carburetor, or with a type of engine with so called single point fuel injection in which fuel is injected by a single fuel injection valve into a point in the intake manifold quite far upstream of the intake ports, because in any case there is already a mist of air-fuel mixture present in the intake ports of the engine whatever is the state of the intake control valve; but in the case of an engine which is equipped with a so called direct fuel injection system in which several fuel injection nozzles are provided much closer to the cylinders of the engine and each squirts injected fuel into the intake manifold quite close upstream of one of the intake ports of the engine incorporating such an intake control valve, then some of the liquid fuel in the squirt of injected fuel tends to strike the intake control valve; and this makes the responsiveness of fuel supply to the engine to be bad. Particularly in the case of operation under low load with a weak mixture, with the air/fuel ratio of the air-fuel mixture being supplied to the engine greater than about 20, then problems occur. It can be practiced to provide an increase of the air/fuel ratio of the air-fuel mixture by an amount depending on the cooling water temperature of the engine, i.e. to provide post-starting fuel increase or choking of the engine; but then when the intake control valve is closed under cold engine running low engine load conditions some of this injected fuel does not get into the combustion chamber of the engine immediately, and may adhere to and around the intake control valve in liquid form and enter the combustion chamber in gobs at various poorly defined later times, with the atomization of the fuel being adversely affected, thus causing irregular engine revolution speed, poor idling and engine faltering, poor drivability, bad responsiveness, low quality of exhaust emissions, and possibly even stalling in the case that the temperature of the engine during the initial running period after starting is less than zero degrees C.